The problem is the initial set up for the summer season. I'm guessing that's at least half a day of unplanned labour? Maybe the councils try to do it, Welwyn Hatfield did last year when it was unseasonably warm... However, that's because I gather they're run by Better and they had staff that were available? But, it wouldn't have cost £0 to open it over the few days, and councils have tight budgets.
But the one a few villages away I think is "pure" council and I think they just have temp/summer staff run them, probably on a 3 months contract or similar.
So, whilst it's a shame, they'll all be open in a couple of weeks anyway - and they're an amazing free community resource that's accessible to pretty much every body.
If you want to take your 3 kids swimming at the local pools, it's around £5 per kid, adults have to be in water with U8 etc. adult tickets are £7+ so, you're easily looking at £25-30 odd for 45-60 mins max.
With the sprinkler parks, granny can come along and watch and paddle if she wants, you can use it as a wheelchair user, babies can have a crawl around, and older kids have a blast, no need for life vests/floatation devices etc. often their in green spaces, so you can stay longer, have picnic s, play frisbee, watch the world gonbu etc.